1. Field of the Invention
This invention relates to a glass edge grinding machine, and more particularly, to a device for selectively controlling peripheral speed of a rotating glass panel during grinding and/or seaming of panel edge portions.
2. Discussion of the Technical Problems
In shaping peripheral edge portions of glass panels, for example, glass panels used for automotive and home windows and for mirrors, the panels are chucked up and rotated about an article center of rotation. One or more grinding wheels powered to rotate about its or their center of rotation(s) is (are) moved into engagement with peripheral edge portions of the glass panel to shape, e.g., grind and/or seam same. Edge grinding machines that operate in a manner similar to that discussed above are taught in the following.
U.S. Pat. Nos: 2,579,337; 3,525,182; 2,597,180; 3,574,976; 2,826,872; 3,621,619; 2,883,800; 3,626,842; 2,906,065; 3,641,711; 2,969,624; 3,827,189; 2,995,876; 4,060,937; 3,274,736; 4,081,927.
In general, the rate of material removal from the panel periphery is selected to prevent "burning" of the glass panel edges which, in severe cases, can result in edge chipping while minimizing the time required for shaping the periphery of the glass panel. Factors that control peripheral material removal rate include rotational speed of the shaping wheel, rotational and peripheral speed of the glass panel, glass panel thickness, grit size of the shaping wheel, biasing force urging the shaping wheel and glass panel edge portions toward one another, peripheral edge configuration of the glass panel and peripheral edge damage of the panel. Usually the shaping wheel speed and characteristics are constant. Of particular interest, therefore, in the following discussion is the effect peripheral edge damage and peripheral configuration of the glass panel have on material removal rate. If the glass panel is symmetrical around its axis of rotation and the peripheral edge damage is essentially uniform, the peripheral edge portions of the glass panel can be shaped using a constant shaping wheel speed and a constant glass panel rotational speed. This is because a symmetrical panel has a constant radius and, therefore, a constant peripheral speed at a constant rotational panel speed and uniform peripheral edge damage has uniform resistance to the removal of material by the shaping wheel. In view of the foregoing, the rotational speeds of the shaping wheel and panel are selected to maximize material removal rate while preventing burning of the panel edges.
Consider now a glass panel that is unsymmetrical around its axis of rotation but has uniform edge damage. In this instance, the peripheral speed of the work plate varies during a cycle of rotation. For example, s the radius of the glass panel, e.g., the distace between the edge portion being ground and the rotational axis of the glass panel increases, the higher the peripheral speed of the glass panel even though the rotation at the center of the glass panel is constant. Since the peripheral speed of the panel varies, an optimum constant material removal rate cannot be attained. This is because selecting a panel speed for material removal rate that prevents burning of the panel edges at the longer radii is usually too slow a panel speed at the shorter radii. On the other hand, a panel speed for optimum material removal rate at the shorter radii usually causes burning of the panel edges at the longer radii. To compensate for unsymmetrical work pieces, the center rotational speed of the panel is varied as the panel passes through consecutive angular increments of rotation. One technique accomplishes this by mounting tripping devices on the outer surface of the rotating shaft supporting the glass panel. The devices are adjusted to control the shaft rotational speed as a function of their corresponding radius, i.e., the distance between the panel axis of rotation and peripheral edge being shaped. In this manner, the rotational speed of the glass panel is varied to vary the peripheral speed of the panel to obtain a substantially constant material removal rate. Techniques similar to the preceding are taught in U.S. Pat. Nos. 3,274,376 and 2,579,337. Although these techniques are acceptable, there are limitations. For example, adjustment of devices for controlling shaft speed requires a new setup for each panel design change. This is time consuming and limits output.
When the glass panel is symmetrical and there is non-uniform peripheral edge damage, e.g., the presence and absence of flares and/or chips at glass panel edge portions and/or varying amounts of material to be removed to attain the desired panel shape, the following problems are usually encountered. The grinding wheel is biased to remove more material at a given time period which puts an extra load on the motor driving the shaping or grinding wheel. If the grinding motor is powered to remove the flared and/or chipped edge portion at a uniform rate, the motor drives the grinding wheel at an increased material removal rate at a chip and/or flare free edge portion. This increased wheel speed can result in burning of the panel edges. In the alternative, if the grinding wheel is powered to remove material from a flare and/or chip free portion, there is increased resistance to material removal at the flared or chipped edge portion. This resistance usually causes the grinding wheel to overheat and can result in damaging the grinding wheel.
Techniques practiced in combination with industrial robots are available for removing material from edges having non-uniform peripheral edge damage and varying amounts of material to be removed for a desired panel configuration. One technique measures the current input to the grinding motor. An increase in current indicates an additional load on the grinding wheel which can be interpreted as a decrease in material removal rate due to increased edge damage. When current increase is sensed, the robot is programmed to move away from the panel edge to take a smaller material bite, thereby decreasing the resistance acting on the wheel and the load on the motor. Although the technique of taking smaller bites of material is acceptable, it requires more workpiece rotation cycles to shape a glass panel to the desired peripheral configuration.
From the above discussion, it can now be appreciated that it would be advantageous to provide a technique for shaping a glass panel that does not have the limitation of the presently available material removal technique.